Decorator for role requirement in Flask - Step-by-Step Execution

Concept Flow - Decorator for role requirement

Define decorator function

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Decorator wraps target function

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On function call: Check user role

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Run function

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Function ends

The decorator wraps a function to check user roles before running it. If the user has the right role, the function runs; otherwise, access is denied.

Execution Sample

Flask

def role_required(role):
    def decorator(func):
        def wrapper(*args, **kwargs):
            if current_user.role == role:
                return func(*args, **kwargs)
            return 'Access Denied', 403
        return wrapper
    return decorator

This code defines a decorator that checks if the current user has the required role before running the decorated function.

Execution Table

Step	Action	Check	Result	Function Call Outcome
1	Call decorated function	N/A	Enter wrapper	Waiting for role check
2	Check current_user.role == 'admin'	current_user.role='admin'	True	Call original function
3	Original function runs	N/A	N/A	Returns function output
4	Call decorated function	N/A	Enter wrapper	Waiting for role check
5	Check current_user.role == 'admin'	current_user.role='guest'	False	Return 'Access Denied', 403

💡 Execution stops after returning function output or access denied response.

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
current_user.role	undefined	'admin'	'guest'	Depends on user
wrapper return	undefined	Function output	'Access Denied', 403	Depends on role check

Key Moments - 2 Insights

Why does the original function only run if the role matches?

Because the wrapper checks current_user.role before calling the original function (see execution_table step 2). If the check fails, it returns access denied instead.

What happens if the user role is not set or different?

The wrapper returns 'Access Denied', 403 immediately without running the original function (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution_table, what is the outcome at step 3 when the role matches?

A The wrapper function exits without calling the original function

B Access Denied error is returned

C The original function runs and returns its output

D An exception is raised

💡 Hint

Check the 'Function Call Outcome' column at step 3 in execution_table

At which step does the role check fail and access is denied?

A Step 5

B Step 4

C Step 2

D Step 1

💡 Hint

Look at the 'Check' and 'Result' columns in execution_table rows for step 5

If current_user.role was 'admin' but the decorator required 'guest', what would happen?

A Function runs normally

B Access Denied is returned

C An error is raised

D The wrapper skips the role check

💡 Hint

Role must match exactly for function to run, see execution_table step 2 logic

Concept Snapshot

Decorator for role requirement:
- Define a decorator that takes a role string
- Inside, define a wrapper function
- Wrapper checks current_user.role against required role
- If match, call original function
- Else, return access denied response
- Use @role_required('role') above route functions

Full Transcript

This visual execution trace shows how a Flask decorator checks user roles before running a function. The decorator wraps the function and on each call, it compares the current user's role to the required role. If they match, the original function runs and returns its output. If not, the wrapper returns an 'Access Denied' message with a 403 status code. The execution table tracks each step: entering the wrapper, checking the role, and either running the function or denying access. The variable tracker shows how current_user.role and the wrapper's return value change during execution. Key moments clarify why the function only runs on role match and what happens if roles differ. The quiz tests understanding of these steps and outcomes. This pattern helps secure Flask routes by role-based access control.